The basic function of a vibration isolator is to isolate, for example, a vehicle structure from input excitations or vibrations generated by an engine or drive train while at the same time adequately supporting these elements for their proper operation within the vehicle. A typical vibration isolator or mount may include a pair of superimposed variable volume, fluid filled chambers separated by a partition. Fluid communication between the chambers during operation of the isolator may occur through an orifice in the partition and/or an elongate inertia track passageway extending between the chambers. Inertia forces generated in the fluid moving between chambers can be utilized to enhance the desired attenuation of vibratory forces. The inertia forces of the oscillating fluid can reduce the dynamic stiffness of the mount at some particular frequency of input excitation, to provide a low stiffness "notch" at a particular excitation frequency where a vibration problem exists. See U.S. patent application Ser. No. 392,089, filed Aug. 8, 1989, and entitled "Mount with Adjustable Length Inertia Track" owned by the assignee of the present invention.
Additional improvement in the operating characteristics of a vibration isolator may be achieved by reducing the overall dynamic stiffness of a vibration isolator for high frequency, low amplitude input excitations while at other times retaining a sufficiently large loss angle or mount "stiffness". As an example, the isolation of low amplitude engine idle oscillations is improved by using a mount of "soft" dynamic stiffness. At the same time, the mount must allow for proper control of the position of the engine and limit the amplitude of movements under static or quasi-static loads such as during acceleration, braking, cornering and torque overloads, etc. Such large engine loads cannot be adequately accommodated by the mount with a low dynamic stiffness to provide a safe, stabilized mounting system.
Two modes of operation in a vibration isolator are provided by a so called fluid decoupler. A decoupler allows for relatively low dynamic stiffness characteristics for high frequency, low amplitude input excitations and other dynamic stiffness characteristics for relatively lower frequency, larger amplitude input excitations. Most decouplers are incorporated in the partition between the opposing fluid chambers of a vibration isolator and oscillate in response to alternate pressurization of the fluid chambers caused by the input excitations. For low input amplitudes, the decoupler will oscillate relatively freely in what is termed the "decoupled state". In this mode, the required low level or soft dynamic stiffness is maintained. For large input amplitudes across the isolator, the fluid pressure will force the decoupler to engage its seat and under these conditions the isolator will be operating in the high dynamic stiffness mode. Exemplary decoupler arrangements are described in U.S. Pat. No. 4,401,897; U.S. Pat. No. 4,469,316; and U.S. Pat. No. 4,742,999.
While known decouplers can be constructed to switch with predictability between the low dynamic stiffness (decoupled) and the high dynamic stiffness (coupled) modes, the particular frequency and amplitude at which this transition occurs may not be readily varied or controlled during operation of the vibration isolator. Moreover, mounts of this type offer limited or no control over the dynamic operating characteristics of the system when in the low dynamic stiffness mode. Another problem associated with existing decouplers is that the transition between the decoupled and coupled modes may be somewhat abrupt which can cause adverse effects resulting from engagement of the decoupling element with the seat.
An effort to achieve improved control capabilities in vibration isolators has been accomplished with the use of electrorheological fluids. Electrorheological fluids exhibit an increase in apparent viscosity when subjected to a high voltage electric field. Mounts which utilize electrorheological fluids to adjust their dynamic stiffness characteristics include those disclosed in U.S. Pat. No. 4,733,758 and U.S. Pat. No. 4,720,087. The mount in U.S. Pat. No. 4,720,087 also includes multiple fixed electrode plates in contact with an electrorheological fluid and enclosed by upper and lower diaphragms. The diaphragms provide "decoupling action" in that they produce minimal damping of small amplitude excitations and large damping of greater amplitude excitations when the diaphragms engage the electrode plates. Because the electrode plates are oriented parallel to the direction of electrorheological fluid movement, it is the yield stress of the electrorheological fluid in shear that dominates the adjustment of the dynamic stiffness. While this arrangement may function satisfactorily for certain applications, it has been found that improved control and greater load carrying capability can be accomplished where the tensile and comprehensive yield stress properties of the electrorheological fluid also determine the dynamic stiffness characteristics of the vibration isolator. Electrorheological fluid yield stress in tension and/or compression is generally three times as great as that in shear and can be utilized to increase the overall stress state achievable per unit volume of fluid in a particular isolator application. Additional control over the movement of elements which provide decoupling action may also be realized when dictated in part by the tensile and compressive yield stress of an electrorheological fluid in surface contact with the decoupling element.
In view of the foregoing, there is a need for a vibration isolator having a electrorheological fluid filled partition assembly which takes advantage of the tensile and compressive properties of the electrorheological fluid in addition to simple shear properties for adjustment of dynamic stiffness characteristics.